Heating elements consisting of a resistive heating wire enclosed is a metal sheath are known in the art. The sheathed resistance heater taught by Naruo et al in U.S. Pat. No. 4,506,251 is typical of such a heating element. This sheathed resistance heater consists of a heating wire coaxially supported in a metal sheath by an electrically insulating powder. Similar heating elements are taught by Neemanns et al in U.S. Pat. No. 4,080,726, Neidhardt et al in U.S. Pat. No. 3,621,204 and Read in U.S. Pat. No. 1,127,281.
The problem with these electric heater elements is that the resistivities of the metal alloys which are resistant to oxidation and corrosion at elevated temperatures are relatively low. Therefore, to achieve the desired resistance, either the diameter of the heater wire must be relatively small or considerable lengths are required. Reducing the diameter of the heater wire makes it relatively sensitive to oxidation or corrosion and therefore subject to failure. Reducing the diameter of the wire heater also increases the surface loading required to achieve the desired radiated heat energy.
Alternatively, increasing the length of the wire to obtain the desired electrical resistance increases the total quantity of the heater wire required which, in turn, increases the cost of the electric heater element. Increasing the length of the wire often results in excessive bulk which may cause packaging problems.
What is needed is a heater element in which the resistivity of the metal heater is effectively increased, permitting shorter lengths and lower surface loading.